When a body of aluminum is melted in a furnace for purposes of casting or the like, dross forms on the surface of the molten aluminum which must be periodically removed, for example by skimming or similar operation. The removed dross contains substantial amounts of free aluminum as well as aluminum oxides, such as bauxite, and certain other metals and metal salts, such as magnesium, manganese and lithium, depending on the nature of the aluminum or aluminum alloy being treated. The dross may also include some nitrides and chlorides, possibly due to the manner in which the dross is treated.
It is recognized in the industry that for economical reasons it is critical to recover in usable form the free aluminum, aluminum oxide, and other by-product metals from the dross. It is also recognized, however, that the recovery of these materials from dross is difficult due, inter alia, to the nature of the dross and the reactivity of aluminum. In a typical recovery process the dross is normally melted at high temperatures in a furnace. However, at elevated temperatures the dross, particularly the free aluminum in the dross, is easily susceptible to oxidation and, moreover, commonly tends to ignite and burn in the presence of air. The burning of the aluminum can decrease substantially the amount of aluminum recovered.
To solve the problems associated with treating dross and improve the efficiency of aluminum recovery, it has been proposed to heat the dross in an induction furnace in the presence of a salt flux. See, for example, McLeod et al. U.S. Pat. No. 3,676,105. The use of a salt flux, which tends to agglomerate the free aluminum, is not desirable because of high costs and in that the salt, which tends to be water-leachable, in turn, must be separated from the aluminum, leading to environmental problems.
Lindsay et al, commonly assigned U.S. Ser. No. 408,388, now U.S. Pat. No. 4,997,476 concurrently filed, describes a process for the recovery of free aluminum and aluminum oxides from aluminum dross comprising heating the dross in a high-temperature rotary furnace using a plasma arc torch, preferably fed with air as the arc gas, without use of an added salt flux. It was found, surprisingly, that the use of a rotary furnace heated with plasma energy from a plasma gun or torch will separate and agglomerate the free aluminum from the dross residue without need for a salt flux. It is preferable that the plasma torch or gun at the time of start-up is directed directly at the charge being melted and subsequently directed towards the walls of the furnace, rather than directly into the charge, in order that the charged dross is heated indirectly by the furnace walls. This indirect heating of the dross eliminates or reduces the nitriding effect when using nitrogen as the plasma torch arc gas, or the formation of oxides when using air as the plasma torch arc gas. Preferably the rotating furnace will also include a tilting mechanism which is advantageous for tapping of the free molten aluminum and removal of solid residue from the furnace.
Lindsay et al also found that in the use of the rotating furnace heated with plasma energy, aluminum oxides--either initially present in the dross or formed during the dross treatment--build up on the walls of the furnace to line the furnace. The free aluminum which melts at a lower temperature than the oxides agglomerates within the interior of the built-up lining where it can easily be removed from the furnace by tilting of the furnace. The built-up aluminum oxide must be periodically removed, for example after each run or after two or three runs, from the walls of the furnace.
As further set forth in Lindsay et al, after the initial treatment of the dross in the rotary furnace and removal of molten free aluminum, it can be desirable to oxidize the non-metal components in the rotary furnace by heating with the plasma arc torch operated on an oxidizing gas such as oxygen to convert the non-metal components to substantially pure metal oxides. In the processes of Lindsay et al, it is desirable that the various stages of the process be automatically sequenced and controlled.